Work machine and method of controlling the same

ABSTRACT

A controller controls a first drive source and a second drive source based on a rotation speed of a right rear wheel measured by a rotation speed sensor and a rotation speed of a left rear wheel measured by a rotation speed sensor, to thereby independently control a rotation speed of each of a right front wheel and a left front wheel.

TECHNICAL FIELD

The present disclosure relates to a work machine and a method ofcontrolling the same.

BACKGROUND ART

US Patent Application Publication No. 2006/0042838 (PTL 1) describesindependently adjusting the speeds of front wheels based on a steeringangle or a combination of steering and articulation angles, in a workmachine such as a motor grader. By independently controlling the rightand left front wheels during a turn of the work machine in this manner,a loss of traction from at least one of the front wheels during the turnis suppressed.

CITATION LIST Patent Literature

PTL 1: US Patent Application Publication No. 2006/0042838

SUMMARY OF INVENTION Technical Problem

However, since the work vehicle described in the above publication ismeant to provide traction during a turn, it is difficult to recover froma skid condition if rear wheels skid during straight line travel.

An object of the present disclosure is to provide a work machine thatreadily recovers from a skid condition even if rear wheels skid duringstraight line travel, and a method of controlling the same.

Solution to Problem

A work machine of the present disclosure includes: a right rear wheel, aleft rear wheel, a right front wheel and a left front wheel; a firstsensor; a second sensor; a first drive source; a second drive source;and a controller. The first sensor measures a rotation speed of theright rear wheel. The second sensor measures a rotation speed of theleft rear wheel. The first drive source provides driving force to theright front wheel. The second drive source provides driving force to theleft front wheel. The controller controls the first drive source and thesecond drive source based on the rotation speed of the right rear wheelmeasured by the first sensor and the rotation speed of the left rearwheel measured by the second sensor, to thereby independently control arotation speed of each of the right front wheel and the left frontwheel.

A method of controlling a work machine of the present disclosure is amethod of controlling a work machine having a right rear wheel, a leftrear wheel, a right front wheel and a left front wheel, and include thefollowing steps.

A rotation speed of the right rear wheel is measured. A rotation speedof the left rear wheel is measured. A rotation speed of each of theright front wheel and the left front wheel is independently controlledbased on the measured rotation speed of the right rear wheel and themeasured rotation speed of the left rear wheel.

Advantageous Effects of Invention

According to the present disclosure, a work machine that readilyrecovers from a skid condition even if rear wheels skid during straightline travel, and a method of controlling the same can be implemented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically showing the construction of amotor grader in one embodiment.

FIG. 2 schematically shows a configuration related to control of driveof running wheels in the motor grader in one embodiment.

FIG. 3 is a functional block diagram showing functions of a controllerin the motor grader in one embodiment.

FIG. 4 is a diagram to illustrate a method of calculating a turningradius of a rear wheel from a rotation speed of the rear wheel.

FIG. 5 is a diagram to illustrate formulas for calculating a turningradius of a front wheel.

FIG. 6 shows a front wheel speed-up ratio table for determining a targetfront wheel speed-up ratio for each of a right front wheel and a leftfront wheel from the turning radius of the rear wheel and anarticulation angle.

FIG. 7 is a flow diagram showing a method of controlling the runningwheels in the motor grader in one embodiment.

DESCRIPTION OF EMBODIMENTS

In the following, a work machine according to an embodiment of thepresent disclosure will be described based on the drawings. The sameelements have the same reference characters allotted in the descriptionbelow and their labels and functions are also the same. Therefore,detailed description thereof will not be repeated.

<Construction of Motor Grader>

Initially, a description will be given of the construction of a motorgrader as an example of a work machine to which the concept of thepresent disclosure is applicable.

FIG. 1 is a perspective view schematically showing the construction of amotor grader in one embodiment. As shown in FIG. 1, a motor grader 1 inthe present embodiment mainly includes running wheels 11, 12, a bodyframe 2, a cab 3, and a work implement 4. Motor grader 1 also includescomponents such as an engine arranged in an engine compartment 6. Workimplement 4 includes a blade 42, for example. Motor grader 1 can do suchworks as land-grading works, snow removal works, and mixing of materialswith blade 42.

In the description of the drawings below, a direction in which motorgrader 1 travels in straight lines is referred to as a fore/aftdirection of motor grader 1. In the fore/aft direction of motor grader1, a side where front wheel 11 is arranged with respect to workimplement 4 is defined as the fore direction. In the fore/aft directionof motor grader 1, a side where rear wheel 12 is arranged with respectto work implement 4 is defined as the aft direction. A lateral directionof motor grader 1 is a direction orthogonal to the fore/aft direction ina plan view. A right side and a left side in the lateral direction infacing front are defined as a right direction and a left direction,respectively. An upward/downward direction of motor grader 1 is adirection orthogonal to the plane defined by the fore/aft direction andthe lateral direction. A side in the upward/downward direction where theground is located is defined as a lower side and a side where the sky islocated is defined as an upper side.

Running wheels 11 and 12 include front wheel 11 and rear wheel 12. Frontwheel 11 includes a right front wheel 11R and a left front wheel 11L oneon each of the right and left sides. Rear wheel 12 includes two rightrear wheels 12R and two left rear wheels 12L (FIG. 2) two on each of theright and left sides. The number and the arrangement of front wheels 11and rear wheels 12 are not limited to the example shown in FIG. 1.

Body frame 2 extends in the fore/aft direction. Body frame 2 includes arear frame 21 and a front frame 22.

Rear frame 21 supports an exterior cover 25 and components such as theengine arranged in engine compartment 6. Exterior cover 25 covers enginecompartment 6. For example, each of four rear wheels 12 is attached torear frame 21. Each of four rear wheels 12 can be rotatably driven bydriving force from the engine.

Front frame 22 is attached in front of rear frame 21. Front frame 22 ispivotably coupled to rear frame 21. Front frame 22 extends in thefore/aft direction. Front frame 22 includes a base end portion coupledto rear frame 21 and a tip end portion opposite to the base end portion.The base end portion of front frame 22 is coupled to a tip end portionof rear frame 21 by means of a vertical central pin. Front frame 22 isformed of a single beam.

An articulation cylinder 23 is attached between front frame 22 and rearframe 21. Front frame 22 is provided as being pivotable (articulatable)with respect to rear frame 21 as a result of extension and retraction ofarticulation cylinder 23.

For example, two front wheels 11 are rotatably attached to the tip endportion of front frame 22. Front wheel 11 is attached as beingrevolvable with respect to front frame 22 as a result of extension andretraction of a steering cylinder 7. Motor grader 1 can change adirection of travel as a result of extension and retraction of steeringcylinder 7.

A counter weight 51 is attached to a front end 2F of body frame 2.Counter weight 51 represents one type of attachments to be attached tofront frame 22. Counter weight 51 is attached to front frame 22 in orderto increase a downward load to be applied to front wheel 11 to allowsteering and to increase a pressing load on blade 42.

Cab 3 is carried on front frame 22, for example. In cab 3, an operationportion (not shown) such as a steering wheel, a gear shift lever, alever for controlling work implement 4, a brake, an accelerator pedal,and an inching pedal is provided. Cab 3 may be carried on rear frame 21.Running wheels 11, 12, body frame 2, and cab 3 form a vehicular body.

Work implement 4 mainly includes a drawbar 40, a swing circle 41, andblade 42, for example. Drawbar 40 is disposed below front frame 22.Drawbar 40 has a front end portion coupled to the tip end portion offront frame 22 by means of a ball bearing portion. The front end portionof drawbar 40 is swingably attached to the tip end portion of frontframe 22.

Drawbar 40 has a rear end portion supported on front frame 22 (a part ofthe vehicular body) by a pair of lift cylinders 44 and 45. As a resultof extension and retraction of the pair of lift cylinders 44 and 45, therear end portion of drawbar 40 can move up and down with respect tofront frame 22. Drawbar 40 is also swingable with an axis extending inthe fore/aft direction being defined as the center, as a result ofextensions and retractions of the pair of lift cylinders 44 and 45 thatare different from each other.

Swing circle 41 is disposed below front frame 22. Swing circle 41 isdisposed below drawbar 40. Swing circle 41 is revolvably (rotatably)attached to the rear end portion of drawbar 40. Swing circle 41 can bedriven by a hydraulic motor 49 as being revolvable clockwise orcounterclockwise with respect to drawbar 40 when viewed from above thevehicle. Blade 42 is provided on swing circle 41. As swing circle 41 isdriven to revolve, a blade angle of blade 42 is adjusted. The bladeangle is an angle of inclination of blade 42 with respect to thefore/aft direction of motor grader 1 when viewed from above the vehicle.

Blade 42 is disposed between front wheel 11 and rear wheel 12. Frontwheel 11 is disposed in front of blade 42. Rear wheel 12 is disposedbehind blade 42. Blade 42 is disposed between front end 2F of body frame2 and a rear end 2R of body frame 2. Blade 42 is supported on swingcircle 41. Blade 42 is supported on drawbar 40 with swing circle 41interposed therebetween. Blade 42 is supported on front frame 22 withdrawbar 40 interposed therebetween.

The pair of lift cylinders 44 and 45 suspends drawbar 40 with respect tothe vehicular body. Specifically, the pair of lift cylinders 44 and 45supports blade 42 located below front frame 22 (a part of the vehicularbody), with drawbar 40 interposed therebetween. As a result of extensionand retraction of the pair of lift cylinders 44 and 45, the heights ofdrawbar 40 and blade 42 can be changed.

As described above, blade 42 is configured to be able to move up anddown with respect to the vehicle, and to be able to swing with the axisextending in the fore/aft direction being defined as the center, withdrawbar 40 and swing circle 41 interposed therebetween.

FIG. 2 schematically shows a configuration related to control of driveof the running wheels in the motor grader in one embodiment. As shown inFIG. 2, motor grader 1 in the present embodiment includes running wheels11, 12, an engine 31, a transmission 32, a final reduction gear 33,tandem devices 34R, 34L, a first drive source 35R, and a second drivesource 35L.

Engine 31 is supported on rear frame 21 shown in FIG. 1. To one outputside of engine 31, final reduction gear 33 is connected withtransmission 32 interposed therebetween. To final reduction gear 33,right tandem device 34R and left tandem device 34L are connected.

To right tandem device 34R, the pair of right rear wheels 12R isconnected. To left tandem device 34L, the pair of left rear wheels 12Lis connected. Engine 31 drives right rear wheels 12R and left rearwheels 12L with transmission 32, final reduction gear 33 and tandemdevices 34R, 34L interposed therebetween.

To the other output side of engine 31, first drive source 35R and seconddrive source 35L are connected. Each of first drive source 35R andsecond drive source 35L is a hydraulic system. First drive source 35Rdrives right front wheel 11R. Second drive source 35L drives left frontwheel 11L.

First drive source 35R includes a right hydraulic pump 35PR and a righthydraulic motor 35MR (first motor). Second drive source 35L includes aleft hydraulic pump 35PL and a left hydraulic motor 35ML (second motor).Each of right hydraulic pump 35PR and left hydraulic pump 35PL is drivenby the output of engine 31 transferred thereto. Each of right hydraulicpump 35PR and left hydraulic pump 35PL is a swash-plate axis pump, forexample.

Right hydraulic motor 35MR is driven with hydraulic oil discharged fromright hydraulic pump 35PR so as to provide driving force to right frontwheel 11R. Left hydraulic motor 35ML is driven with hydraulic oildischarged from left hydraulic pump 35PL so as to provide driving forceto left front wheel 11L. Each of hydraulic motors 35MR and 35ML is aswash-plate axis motor, for example. Note that each of hydraulic motors35MR and 35ML may be a radial-piston motor.

Motor grader 1 in the present embodiment further includes rotation speedsensors 36R, 36L, a speed sensor 36V, solenoids 37R, 37L, 38R, 38L, anarticulation sensor 39, and a controller 50.

Rotation speed sensor 36R (first sensor) detects a rotation speed ofright rear wheels 12R, and generates a signal of the rotation speed ofright rear wheels 12R. The signal of the rotation speed of right rearwheels 12R generated at rotation speed sensor 36R is output tocontroller 50. Rotation speed sensor 36L (second sensor) detects arotation speed of left rear wheels 12L, and generates a signal of therotation speed of left rear wheels 12L. The signal of the rotation speedof left rear wheels 12L generated at rotation speed sensor 36L is outputto controller 50.

Rotation speed sensor 36R measures a rotation speed of a rotating shaftin right tandem device 34R, for example. Rotation speed sensor 36Lmeasures a rotation speed of a rotating shaft in left tandem device 34L,for example.

Speed sensor 36V detects a moving speed (traveling speed) duringmovement (during travel) of motor grader 1, and generates a signal ofthe moving speed. The signal of the moving speed generated at speedsensor 36V is output to controller 50. Speed sensor 36V measures arotation speed of an output shaft of transmission 32, for example. Speedsensor 36V may detect the moving speed of motor grader 1 using a GPS(Global Positioning System), for example.

Right pump solenoid 37R controls an amount of hydraulic oil dischargedfrom right hydraulic pump 35PR to right hydraulic motor 35MR, bychanging the angle of a pump swash plate in right hydraulic pump 35PR.Left pump solenoid 37L controls an amount of hydraulic oil dischargedfrom left hydraulic pump 35PL to left hydraulic motor 35ML, by changingthe angle of a pump swash plate in left hydraulic pump 35PL. Solenoids37R and 37L control hydraulic pumps 35PR and 35PL, respectively, basedon control signals provided from controller 50.

Right motor solenoid 38R controls a rotation speed of right front wheel11R by changing the angle of a motor swash plate in right hydraulicmotor 35MR. Left motor solenoid 38L controls a rotation speed of leftfront wheel 11L by changing the angle of a motor swash plate in lefthydraulic motor 35ML. Solenoids 38R and 38L control hydraulic motors35MR and 35ML, respectively, based on control signals provided fromcontroller 50.

Note that if each of hydraulic motors 35MR and 35ML is a radial-pistonmotor, the rotation speeds of front wheels 11R and 11L are controlled bychanging the number of pistons to which the hydraulic oil is supplied,out of a plurality of circumferentially disposed and radially extendingpistons.

Articulation sensor 39 detects an articulation angle (coupling angle)between front frame 22 and rear frame 21, and generates an articulationangle signal. The articulation angle signal generated at articulationsensor 39 is output to controller 50.

<Configuration of Controller 50>

FIG. 3 is a functional block diagram showing functions of controller 50in the motor grader in one embodiment. As shown in FIG. 3, controller 50includes a rear wheel turning radius calculation unit 50 a, a frontwheel turning radius calculation unit 50 b, a target front wheelrotation speed calculation unit 50 c, a right front wheel rotation speedcommand unit 50 dR, a left front wheel rotation speed command unit 50dL, and a memory 50 e.

Based on the rotation speed of right rear wheels 12R measured atrotation speed sensor 36R and the rotation speed of left rear wheels 12Lmeasured at rotation speed sensor 36L, rear wheel turning radiuscalculation unit 50 a calculates a turning radius of rear wheel 12including right rear wheels 12R and left rear wheels 12L. Rear wheelturning radius calculation unit 50 a outputs the calculated turningradius of rear wheel 12 to front wheel turning radius calculation unit50 b.

Based on the turning radius of rear wheel 12 calculated by rear wheelturning radius calculation unit 50 a, and the articulation angle sensedby articulation sensor 39, front wheel turning radius calculation unit50 b calculates a turning radius of right front wheel 11R and a turningradius of left front wheel 11L. Based on the calculated turning radiusof each of right front wheel 11R and left front wheel 11L, the turningradius of rear wheel 12 calculated by rear wheel turning radiuscalculation unit 50 a, and the articulation angle sensed by articulationsensor 39, front wheel turning radius calculation unit 50 b creates afront wheel speed-up ratio table (FIG. 6). Front wheel turning radiuscalculation unit 50 b outputs the created front wheel speed-up ratiotable to memory 50 e.

Memory 50 e stores the front wheel speed-up ratio table. The front wheelspeed-up ratio table defines a front wheel speed-up ratio correspondingto a rear wheel turning radius and an articulation angle, as shown inFIG. 6, for example. The details of this front wheel speed-up ratiotable will be described later. The front wheel speed-up ratio is a valueobtained by dividing a front wheel turning radius by a rear wheelturning radius (front wheel turning radius/rear wheel turning radius).This memory 50 e may be located outside of controller 50.

Target front wheel rotation speed calculation unit 50 c calculates atarget front wheel rotation speed of each of right front wheel 11R andleft front wheel 11L. Specifically, this is performed as describedbelow.

Target front wheel rotation speed calculation unit 50 c refers to thefront wheel speed-up ratio table stored in memory 50 e, and based on theturning radius of rear wheel 12 calculated by rear wheel turning radiuscalculation unit 50 a, and the articulation angle sensed by articulationsensor 39, determines a front wheel speed-up ratio as a target for eachof right front wheel 11R and left front wheel 11L (target front wheelspeed-up ratio).

Based on a rear wheel rotation speed, and the target front wheelspeed-up ratio for each of right front wheel 11R and left front wheel11L calculated as above, target front wheel rotation speed calculationunit 50 c calculates a rotation speed as a target for each of rightfront wheel 11R and left front wheel 11L (target front wheel rotationspeed).

The rear wheel rotation speed may be, for example, the rotation speed ofthe output shaft of transmission 32 measured by speed sensor 36V.Alternatively, the rear wheel rotation speed may be, for example, arotation speed of rear wheel 12 calculated from the moving speed ofmotor grader 1 measured using a GPS by speed sensor 36V. Alternatively,the rear wheel rotation speed may be an average of the rotation speed ofright rear wheels 12R measured by rotation speed sensor 36R and therotation speed of left rear wheels 12L measured by rotation speed sensor36L.

A signal of the rotation speed of the output shaft of transmission 32measured by speed sensor 36V, or a signal of each of the rotation speedof right rear wheels 12R measured by rotation speed sensor 36R and therotation speed of left rear wheels 12L measured by rotation speed sensor36L may be output to memory 50 e.

In this case, these signals are output to target front wheel rotationspeed calculation unit 50 c through memory 50 e, and used forcalculation of the target front wheel rotation speed at target frontwheel rotation speed calculation unit 50 c. Alternatively, these signalsmay be output directly to target front wheel rotation speed calculationunit 50 c and used for calculation of the target front wheel rotationspeed at target front wheel rotation speed calculation unit 50 c,without being output to memory 50 e.

Target front wheel rotation speed calculation unit 50 c outputs thecalculated target front wheel rotation speed of right front wheel 11R toright front wheel rotation speed command unit 50 dR. Target front wheelrotation speed calculation unit 50 c also outputs the calculated targetfront wheel rotation speed of left front wheel 11L to left front wheelrotation speed command unit 50 dL.

Based on the target front wheel rotation speed signal of right frontwheel 11R output from target front wheel rotation speed calculation unit50 c, right front wheel rotation speed command unit 50 dR controls firstdrive source 35R of right front wheel 11R. Based on the target frontwheel rotation speed signal of left front wheel 11L output from targetfront wheel rotation speed calculation unit 50 c, left front wheelrotation speed command unit 50 dL controls second drive source 35L ofleft front wheel 11L.

As stated above, controller 50 controls first drive source 35R andsecond drive source 35L based on the rotation speed of right rear wheels12R measured at rotation speed sensor 36R and the rotation speed of leftrear wheels 12L measured at rotation speed sensor 36L, to therebyindependently control the rotation speed of each of right front wheel11R and left front wheel 11L.

<Method of Calculating Turning Radius of Rear Wheel and Turning Radiusof Front Wheel>

A method of calculating a turning radius of a rear wheel and a turningradius of a front wheel is now described with reference to FIGS. 4 and5.

FIG. 4 is a diagram to illustrate a method of calculating a turningradius of a rear wheel from a rotation speed of the rear wheel. FIG. 5is a diagram to illustrate formulas for calculating a turning radius ofa front wheel.

As shown in FIG. 4, when a turning radius of a center (rear wheelcenter) C1 of right rear wheels 12R and left rear wheels 12L is R, arotation speed of right rear wheels 12R is v_(r), a rotation speed ofleft rear wheels 12L is v_(l), and a tread width between right rearwheels 12R and left rear wheels 12L is Tr, then turning radius R duringa right turn is expressed in the following Formula 1, and turning radiusR during a left turn is expressed in the following Formula 2:

$\begin{matrix}{R = {{v_{r} \cdot \frac{T_{r}}{v_{l} - v_{r}}} + \frac{T_{r}}{2}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \\{R = {{v_{l} \cdot \frac{T_{r}}{v_{r} - v_{l}}} + \frac{T_{r}}{2}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

During a right turn, turning radius R of rear wheel center C1 during theright turn is calculated by substitution of rotation speed v_(r) ofright rear wheels 12R measured at rotation speed sensor 36R and rotationspeed v_(l) of left rear wheels 12L measured at rotation speed sensor36L into Formula 1 described above.

During a left turn, turning radius R of rear wheel center C1 during theleft turn is calculated by substitution of rotation speed v_(r) of rightrear wheels 12R measured at rotation speed sensor 36R and rotation speedv_(l) of left rear wheels 12L measured at rotation speed sensor 36L intoFormula 2 described above.

A turning radius R_(r) of right front wheel 11R is calculated bysubstitution of turning radius R of rear wheel center C1 into thefollowing Formula 3, and a turning radius R_(l) of left front wheel 11Lis calculated by substitution of turning radius R of rear wheel centerC1 into the following Formula 4:

R _(r)=√{square root over ((R−x _(r))² +y _(r) ²)}  [Formula 3]

R _(l)=√{square root over ((R−x _(l))² +y _(l) ²)}  [Formula 4]

Note that turning radius R in Formula 3 and Formula 4 described above isturning radius R calculated in Formula 1 during a right turn, and isturning radius R calculated in Formula 2 during a left turn.

In addition, x_(r), y_(r), x_(l) and y_(l) indicated in Formula 3 andFormula 4 described above are expressed in the following Formula 5,Formula 6, Formula 7 and Formula 8, respectively:

$\begin{matrix}{x_{r} = {{L_{2}\sin \theta_{a}} + {\frac{L_{3}}{2}\cos \theta_{a}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack \\{y_{r} = {L_{1} + {L_{2}\cos \theta_{a}} - {\frac{L_{3}}{2}\sin \theta_{a}}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack \\{x_{l} = {{L_{2}\sin \theta_{a}} - {\frac{L_{3}}{2}\cos \theta_{a}}}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack \\{y_{l} = {L_{1} + {L_{2}\cos \theta_{a}} + {\frac{L_{3}}{2}\sin \theta_{a}}}} & \left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

L₁ in Formula 5, Formula 6, Formula 7 and Formula 8 described aboverepresents a distance between rear wheel center C1 and an articulationcenter 24, as shown in FIG. 5. L₂ represents a distance betweenarticulation center 24 and a front accelerator central pin. L₃represents a tread width between right front wheel 11R and left frontwheel 11L. In addition, θ_(a) represents an articulation angle. Each ofL₁, L₂ and L₃ is a numerical value determined by the type of workmachine 1.

As stated above, turning radius R of rear wheel center C1 is calculatedusing Formula 1 or Formula 2 described above. Turning radius R_(r) ofright front wheel 11R is calculated using Formula 3 described above.Turning radius R_(l) of left front wheel 11L is calculated using Formula4 described above.

<Front Wheel Speed-Up Ratio Table>

The front wheel speed-up ratio table is now described with reference toFIG. 6.

FIG. 6 shows the front wheel speed-up ratio table for determining afront wheel speed-up ratio for each of the right and left front wheelsfrom a turning radius of a rear wheel and an articulation angle. Thefront wheel speed-up ratio table shown in FIG. 6 defines relation amongthe turning radius of the rear wheel, the articulation angle, and thefront wheel speed-up ratio.

The item of “rear wheel turning radius” in the front wheel speed-upratio table is divided into items of “right turn,” “neutral (straightline travel)” and “left turn.” The “right turn” item is divided intofour items of “small,” “↑”, “↓” and “large,” for example. The “large”item of the “right turn” defines a relatively large numerical value ofturning radius during a right turn, and the “small” item defines arelatively small numerical value of turning radius during a right turn.Each of “↑” and “↓” of the “right turn” defines a numerical valuesmaller than that of the “large” of the turning radius and larger thanthat of the “small” of the turning radius. In addition, “↑” of the“right turn” defines a numerical value closer to the “small” of the“right turn” than “↓” of the “right turn.”

The “left turn” item is divided into four items of “small,” “↑”, “↓” and“large,” for example. The “large” item of the “left turn” defines arelatively large numerical value of turning radius during a left turn,and the “small” item defines a relatively small numerical value ofturning radius during a left turn. Each of “↑” and “↓” of the “leftturn” defines a numerical value smaller than that of the “large” of theturning radius and larger than that of the “small” of the turningradius. In addition, “↑” of the “left turn” defines a numerical valuecloser to the “large” of the “left turn” than “↓” of the “left turn.”

The item of “articulation angle” is divided into items of “leftarticulation,” “neutral” and “right articulation.” The “leftarticulation” item is divided into three items of “small,” “middle” and“large,” for example. The “large” item of the “left articulation”defines a relatively large articulation angle during a leftarticulation. The “small” item of the “left articulation” defines arelatively small articulation angle during a left articulation. The“middle” item of the “left articulation” defines an articulation anglebetween the “large” and the “small” of the “left articulation.”

The “right articulation” item is divided into three items of “small,”“middle” and “large,” for example. The “large” item of the “rightarticulation” defines a relatively large articulation angle during aright articulation. The “small” item of the “right articulation” definesa relatively small articulation angle during a right articulation. The“middle” item of the “right articulation” defines an articulation anglebetween the “large” and the “small” of the “right articulation.”

A numerical value of “1.00” in the front wheel speed-up ratio tableshown in FIG. 6 is a target front wheel speed-up ratio. Each of L01 toL03, L11 to L13, L21 to L23, L31 to L34, R01 to R03, R11 to R13, R21 toR23, and R31 to R34 defines a numerical value of the target front wheelspeed-up ratio corresponding to each machine type.

This front wheel speed-up ratio table shown in FIG. 6 is created forboth right front wheel 11R and left front wheel 11L, and stored inmemory 50 e shown in FIG. 3. While the “right turn” and “left turn” areeach divided into four items in the front wheel speed-up ratio tableshown in FIG. 6, they may be divided into three or less items, or fiveor more items. While the items of “left articulation” and “rightarticulation” are each divided into three items, they may be dividedinto two or less items, or four or more items.

<Creation of Front Wheel Speed-Up Ratio Table>

The front wheel speed-up ratio table shown in FIG. 6 is created based ona rear wheel turning radius calculated by rear wheel turning radiuscalculation unit 50 a shown in FIG. 3, an articulation angle measured byarticulation sensor 39, and a front wheel turning radius calculated byfront wheel turning radius calculation unit 50 b. Specifically, thetable is created from the rear wheel turning radius, the articulationangle, and a front wheel speed-up ratio calculated by dividing the frontwheel turning radius by the rear wheel turning radius.

This front wheel speed-up ratio table is created at front wheel turningradius calculation unit 50 b shown in FIG. 3, and once created, isoutput from front wheel turning radius calculation unit 50 b to memory50 e, and stored in memory 50 e.

<Method of Calculating Target Front Wheel Rotation Speed>

A method of calculating a target front wheel rotation speed is nowdescribed.

The calculation of the target front wheel rotation speed is performed attarget front wheel rotation speed calculation unit 50 c shown in FIG. 3.This target front wheel rotation speed calculation unit 50 c refers tothe front wheel speed-up ratio table (FIG. 6) stored in memory 50 e, andbased on the turning radius of rear wheel 12 calculated by rear wheelturning radius calculation unit 50 a, and the articulation anglemeasured by articulation sensor 39, determines a target front wheelspeed-up ratio for each of right front wheel 11R and left front wheel11L. Specifically, when the articulation angle of motor grader 1 shownin FIGS. 4 and 5 is “large” of the “right articulation” shown in FIG. 6,for example, and the rear wheel turning radius is “large” of the “rightturn” shown in FIG. 6, for example, then the target front wheel speed-upratio is determined to be a numerical value defined in “R31”.

The target front wheel speed-up ratio determined as described above is avalue obtained by dividing a front wheel turning radius as a target(target front wheel turning radius) by the rear wheel turning radius.

While a method of calculating the target front wheel rotation speedusing the front wheel speed-up ratio table has been described above, thetarget front wheel rotation speed may be calculated without using thefront wheel speed-up ratio table.

<Method of Controlling Running Wheels 11 and 12>

A method of controlling running wheels 11 and 12 in the motor grader inone embodiment is now described with reference to FIG. 7.

FIG. 7 is a flow diagram showing the method of controlling runningwheels 11 and 12 in the motor grader in one embodiment. As shown inFIGS. 3 and 7, a rotation speed of right rear wheels 12R is measured byrotation speed sensor 36R (step S1 a: FIG. 7). A rotation speed of leftrear wheels 12L is measured by rotation speed sensor 36L (step S1 b:FIG. 7). An articulation angle is measured by articulation sensor 39(step S1 c: FIG. 7).

Then, as shown in FIG. 3, a signal of the rotation speed measured ateach of rotation speed sensors 36R and 36L is output to rear wheelturning radius calculation unit 50 a of controller 50.

In rear wheel turning radius calculation unit 50 a, a turning radius ofthe rear wheel is calculated (step S2: FIG. 7). Specifically, during aright turn, turning radius R of rear wheel center C1 during the rightturn is calculated by substitution of rotation speed v_(r) of right rearwheels 12R measured at rotation speed sensor 36R and rotation speedv_(l) of left rear wheels 12L measured at rotation speed sensor 36L intoFormula 1 described above.

During a left turn, turning radius R of rear wheel center C1 during theleft turn is calculated by substitution of rotation speed v_(r) of rightrear wheels 12R measured at rotation speed sensor 36R and rotation speedv_(l) of left rear wheels 12L measured at rotation speed sensor 36L intoFormula 2 described above.

Then, as shown in FIG. 3, the turning radius of rear wheel 12 calculatedat rear wheel turning radius calculation unit 50 a is output to targetfront wheel rotation speed calculation unit 50 c. The articulation anglemeasured by articulation sensor 39 is also output to target front wheelrotation speed calculation unit 50 c.

In target front wheel rotation speed calculation unit 50 c, a targetfront wheel rotation speed is determined by reference to the front wheelspeed-up ratio table stored in memory 50 e (step S3: FIG. 7).Specifically, target front wheel rotation speed calculation unit 50 crefers to the front wheel speed-up ratio table stored in memory 50 e,and based on the turning radius of rear wheel 12 calculated by rearwheel turning radius calculation unit 50 a, and the articulation anglesensed by articulation sensor 39, determines a target front wheelspeed-up ratio for each of right front wheel 11R and left front wheel11L.

The target front wheel speed-up ratio determined as described above is avalue obtained by dividing a target front wheel turning radius by a rearwheel turning radius. Thus, the target front wheel rotation speed iscalculated based on the target front wheel speed-up ratio and the rearwheel rotation speed.

The target front wheel rotation speed of right front wheel 11Rcalculated by target front wheel rotation speed calculation unit 50 c isoutput to right front wheel rotation speed command unit 50 dR. Thetarget front wheel rotation speed of left front wheel 11L calculated bytarget front wheel rotation speed calculation unit 50 c is output toleft front wheel rotation speed command unit 50 dL.

Right front wheel rotation speed command unit 50 dR controls a rotationspeed of right front wheel 11R based on the target front wheel rotationspeed of right front wheel 11R, and left front wheel rotation speedcommand unit 50 dL controls a rotation speed of left front wheel 11Lbased on the target front wheel rotation speed of left front wheel 11L(step S4: FIG. 7).

Specifically, right front wheel rotation speed command unit 50 dRoutputs a signal for controlling the rotation of right front wheel 11Rto at least one of right pump solenoid 37R and right motor solenoid 38Rshown in FIG. 2. Right pump solenoid 37R that has received this signalchanges the angle of the pump swash plate of right hydraulic pump 35PR.Right motor solenoid 38R that has received this signal changes the angleof the motor swash plate of right hydraulic motor 35MR. The rotationspeed of right front wheel 11R is controlled by the change in angle ofat least one of the pump swash plate of right hydraulic pump 35PR andthe motor swash plate of right hydraulic motor 35MR.

Left front wheel rotation speed command unit 50 dL outputs a signal forcontrolling the rotation of left front wheel 11L to at least one of leftpump solenoid 37L and left motor solenoid 38L shown in FIG. 2. Left pumpsolenoid 37L that has received this signal changes the angle of the pumpswash plate of left hydraulic pump 35PL. Left motor solenoid 38L thathas received this signal changes the angle of the motor swash plate ofleft hydraulic motor 35ML. The rotation speed of left front wheel 11L iscontrolled by the change in angle of at least one of the pump swashplate of left hydraulic pump 35PL and the motor swash plate of lefthydraulic motor 35ML.

As stated above, the rotation speed of right front wheel 11R and therotation speed of left front wheel 11L are independently controlledbased on the rotation speed of rear wheel 12.

Advantageous effects of the present embodiment are now described.

In the present embodiment, as shown in FIG. 2, the rotation speeds ofrear wheels 12R and 12L are measured at rotation speed sensors 36R and36L. Thus, even if either right rear wheels 12R or left rear wheels 12Lskid during straight line travel, change in the rotation speeds of rearwheels 12R and 12L due to the skid can be sensed by rotation speedsensors 36R and 36L.

Moreover, in the present embodiment, as shown in FIG. 2, the rotationspeed of right front wheel 11R and the rotation speed of left frontwheel 11L are independently controlled based on the rotation speeds ofrear wheels 12R and 12L. Thus, even if the rotation speeds of rearwheels 12R and 12L change due to a skid, the rotation speed of rightfront wheel 11R and the rotation speed of left front wheel 11L can beindependently controlled accordingly. Recovery from the skid of rearwheels 12R and 12L can thereby be readily performed. If right rearwheels 12R skid, for example, the rotation speed of right front wheel11R can be increased with respect to the rotation speed of left frontwheel 11L, so that recovery from the skid condition can be readilyperformed.

Moreover, in the present embodiment, as shown in FIG. 3, controller 50causes rear wheel turning radius calculation unit 50 a to calculate theturning radius of rear wheel 12 including right rear wheels 12R and leftrear wheels 12L based on the rotation speed of right rear wheels 12Rmeasured at rotation speed sensor 36R and the rotation speed of leftrear wheels 12L measured at rotation speed sensor 36L. Based on thiscalculated turning radius of rear wheel 12, the target front wheelrotation speed of each of right front wheel 11R and left front wheel 11Lcan be calculated.

Moreover, in the present embodiment, as shown in FIG. 3, controller 50causes target front wheel rotation speed calculation unit 50 c tocalculate the target front wheel turning radius of right front wheel 11Rand the target front wheel turning radius of left front wheel 11L basedon the calculated turning radius of rear wheel 12, and the articulationangle detected by articulation sensor 39. Based on the target frontwheel turning radius of right front wheel 11R and the target front wheelturning radius of left front wheel 11L thus calculated, the target frontwheel rotation speed of each of right front wheel 11R and left frontwheel 11L can be calculated.

Moreover, in the present embodiment, as shown in FIG. 3, controller 50causes right front wheel rotation speed command unit 50 dR to controlfirst drive source 35R so as to attain the target front wheel rotationspeed of right front wheel 11R, and causes left front wheel rotationspeed command unit 50 dL to control second drive source 35L so as toattain the target front wheel rotation speed of left front wheel 11L.The rotation speed of each of right front wheel 11R and left front wheel11L is thereby independently controlled, so that recovery from a skidcondition can be readily performed.

Moreover, in the present embodiment, as shown in FIG. 2, first drivesource 35R includes right hydraulic motor 35MR, and second drive source35L includes left hydraulic motor 35ML. The rotation speed of rightfront wheel 11R is thereby controlled by right hydraulic motor 35MR, andthe rotation speed of left front wheel 11L is thereby controlled by lefthydraulic motor 35ML. Thus, the rotation speed of right front wheel 11Rand the rotation speed of left front wheel 11L can be independentlycontrolled.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than thedescription above, and is intended to include any modifications withinthe meaning and scope equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1 motor grader; 2 body frame; 2F front end; 2R rear end; 3 cab; 4 workimplement; 6 engine compartment; 7 steering cylinder; 11 front wheel;11L left front wheel; 11R right front wheel; 12 rear wheel; 12L leftrear wheel; 12R right rear wheel; 21 rear frame; 22 front frame; 23articulation cylinder; 24 articulation center; 25 exterior cover; 31engine; 32 transmission; 33 final reduction gear; 34L left tandemdevice; 34R right tandem device; 35L second drive source; 35ML lefthydraulic motor; 35MR right hydraulic motor; 35PL left hydraulic pump;35PR right hydraulic pump; 35R first drive source; 36L, 36R rotationspeed sensor; 37L left pump solenoid; 37R right pump solenoid; 38L leftmotor solenoid; 38R right motor solenoid; 39 articulation sensor; 40drawbar; 41 swing circle; 42 blade; 44 lift cylinder; 49 hydraulicmotor; 50 controller; 50 a rear wheel turning radius calculation unit;50 b front wheel turning radius calculation unit; 50 c target frontwheel rotation speed calculation unit; 50 dL left front wheel rotationspeed command unit; 50 dR right front wheel rotation speed command unit;50 e memory; 51 counter weight; C1 rear wheel center.

1. A work machine comprising: a right rear wheel, a left rear wheel, aright front wheel and a left front wheel; a first sensor that measures arotation speed of the right rear wheel; a second sensor that measures arotation speed of the left rear wheel; a first drive source thatprovides driving force to the right front wheel; a second drive sourcethat provides driving force to the left front wheel; and a controllerthat controls the first drive source and the second drive source basedon the rotation speed of the right rear wheel measured by the firstsensor and the rotation speed of the left rear wheel measured by thesecond sensor, to thereby independently control a rotation speed of eachof the right front wheel and the left front wheel.
 2. The work machineaccording to claim 1, wherein the controller calculates a turning radiusof rear wheels including the right rear wheel and the left rear wheelbased on the rotation speed of the right rear wheel measured by thefirst sensor and the rotation speed of the left rear wheel measured bythe second sensor.
 3. The work machine according to claim 2, furthercomprising: a front frame provided with the right front wheel and theleft front wheel; a rear frame provided with the right rear wheel andthe left rear wheel and coupled to the front frame; and an articulationsensor that detects an articulation angle between the front frame andthe rear frame, wherein the controller determines a target front wheelrotation speed of the right front wheel and a target front wheelrotation speed of the left front wheel based on the calculated turningradius of the rear wheels and the articulation angle detected by thearticulation sensor.
 4. The work machine according to claim 3, whereinthe controller controls the first drive source so as to attain thetarget front wheel rotation speed of the right front wheel, and controlsthe second drive source so as to attain the target front wheel rotationspeed of the left front wheel.
 5. The work machine according to claim 1,wherein the first drive source includes a first motor, and the seconddrive source includes a second motor.
 6. A method of controlling a workmachine having a right rear wheel, a left rear wheel, a right frontwheel and a left front wheel, the method comprising: measuring arotation speed of the right rear wheel; measuring a rotation speed ofthe left rear wheel; and independently controlling a rotation speed ofeach of the right front wheel and the left front wheel based on themeasured rotation speed of the right rear wheel and the measuredrotation speed of the left rear wheel.